Function uniqueByValue

Filters the input iterable, only yielding unique values. Use unique to compare by object reference instead.

Streaming: Works with unbounded iterables.

const d = ['a', 'b', 'c', 'b', 'd' ];
for (const v of uniqueByValue(d)) {
 // Yields: 'a', 'b', 'c', 'd'
// (extra 'b' is skipped)
}
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By default, JSON.stringify is used to create a string representing value. These are added to a Set of strings, which is how we keep track of uniqueness. If the value is already a string it is used as-is.

This allows you to have custom logic for what determines uniqueness. Eg, using a single field of an object as an identifier:

const people = [
 { name: `Mary`, size: 20 }, { name: `Abdul`, size: 19 }, { name: `Mary`, size: 5 }
]
for (const v of uniqueByValue(d, v=>v.name)) {
 // Yields: { name: `Mary`, size: 20 }, { name: `Abdul`, size: 19 }
 // Second 'Mary' is skipped because name is the same, even though size field is different.
}
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If you want to keep track of the set of keys, or prime it with some existing data, provide a Set instance:

const unique = new Set();
unique.add(`b`);
const d = [`a`, `b`, `c`];
for (const v of uniqueByValue(d, toStringDefault, unique)) {
 // Yields: `a`, `c`
 // `b` is skipped because it was already in set
}
// After completion, `unique` contains `a`, `b` and `c`.
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Creating your own Set is useful for tracking unique values across several calls to uniqueByValue.

Param: input

Param: seen

Param: toString